There's no way to observe any world but our own, of course, but the theories have gained a measure of popularity, especially among atheistic scientists, because if there really are a near infinite number of worlds then the probability of a world like ours existing goes from almost nil to inevitable. Otherwise, if ours is the only world there is, its extraordinary fine-tuning makes it exceedingly unlikely to have happened apart from the intervention of an intelligent agent. This is an unacceptable state of affairs for an atheist, thus the concept of the multiverse is seen as something of an intellectual lifesaver.
Horgan writes:
These multiverse theories all share the same fundamental defect: They can be neither confirmed nor falsified. Hence, they don't deserve to be called scientific, according to the well-known criterion proposed by the philosopher Karl Popper. Some defenders of multiverses and strings mock skeptics who raise the issue of falsification as "Popperazi" - which is cute but not a counterargument. Multiverse theories aren't theories - they're science fictions, theologies, works of the imagination unconstrained by evidence.Horgan goes so far as to call such speculations immoral because they divert scientists from the crucial task of saving the world from serious threats.
I think it's nonsense for Horgan, who is himself an atheist, to be making moral judgments, but I do agree with what he says in the quoted passage. Speculation about multiverses is not science, its metaphysics. There's no way such universes could be empirically observed or tested for which is the sine qua non for science.
The irony is that although many scientists agree that such "theologies" aren't genuine science no one, as far as I know, complains much about anybody talking about them in a physics class. Yet let someone introduce intelligent design into a biology class, and the screeching and howling from the defenders of scientific purity is ear-piercing. ID is not testable, they protest, it's not falsifiable, it's not science, it's theology, etc. All of these asseverations are dubious, but let's grant them. Why then is ID verboten but multiverse speculations are not?
For that matter, why is the Darwinian claim that natural processes are sufficient to produce life in all its variety admissable, but the basic ID claim that natural processes are not adequate to produce complex, specified information is not?
Of course, we know the answer. Multiverses and Darwinism both provide intellectual cover for atheistic naturalism and are thus welcome. Intelligent Design, however, is compatible with theism and is thus to be banned, proscribed, censored, and exiled at all costs.
The debate, in other words, is not about science, it's about which metaphysical beliefs our children will be suffered to hear in their classrooms. Count me among the "Popperazi", but also count me among those who think that all such speculations should be permitted in the science classroom as long as the teacher wants to present them and as long as there's time for the rest of the curriculum to be taught.